1. Field of the Invention
This invention relates to methods of and apparatus for the generation of split-screen video displays, especially for use in video conferencing, wherein each conferee has the capability of seeing, for example, up to four or five remote separate camera sources on a single monitor. Accordingly, it is a general object of this invention to provide new and improved methods and apparatus of such character.
2. General Background
The method previously in use in the prior art for the generation of a split-screen image was a part of a "special effects" capability in a TV production facility. The requirements to be met to compose such a split-screen image were as follows:
(1) Camera synchronization was to be achieved by reception of a reference composite synchronization signal transmitted from the central equipment location to each camera.
(2) The setting of delay switches at each camera location was required to even up delays from each camera to a central site. This requirement was only necessary when camera distances varied by more than about 300 meters.
(3) At each camera site delay had to be further adjusted to an accuracy of about 1 ns to retain color hue. The maximum variation of this control was about 400 ns.
(4) Before an operator could compose a split-screen picture, each camera had to be mechanically panned and tilted to position the desired image in the required area of the screen to correspond with its appearance in the split-screen display.
The above requirements meant that the received horizontal and vertical synchronization signals from the cameras were nearly time coincident. In addition, the color subcarrier burst phases were synchronized within about 1.degree.. This meant that by using fast analog switches, each picture could be displayed in turn as required on a split-screen display.
The disadvantages of such prior techniques, especially for "switched" video conferences (e.g., a conference of four set up from a possible 10 sites), include:
(1) The necessity to distribute a wideband composite synchronization signal to all cameras (usually via video cable).
(2) The requirement that camera connection to the split-screen equipment had to be initially set up to achieve close line and field synchronization and precise color phase (1 ns), related to the central master, at the split-screen generator.
(3) The necessity to mechanically pan and tilt cameras to position prime subject material into the appropriate area of the screen for split-screen composition. Such panning and tilting was usually not convenient in office-to-office video conferencing where cameras were usually mechanically fixed.
(4) The required use of edges of camera images which might not be as sharp or well converged as the central portion. (Particularly applicable to the more economical color cameras used for video conferencing.)
(5) In some video conferencing systems, a single camera was used for both full screen (e.g., graphics or person) and split-screen modes. In this case it was desirable that the camera did not need to be mechanically panned and tilted between a full screen and split-screen pictures, as this interrupted the continuity of the conference since a significant time was involved.